1. Field of the Invention
This invention relates to polymers and specifically to rigid polyurethane foams. Even more specifically, the invention relates to open celled rigid polyurethane foams and to methods and compositions for their preparation. The invention further relates to the use of such foams as insulation materials.
2. Description of the Related Art
In the manufacture of refrigeration cabinets, picnic coolers, doors, and other insulated containers, polyurethane foam is poured in place between two substrates defining a cavity.
There are several desirable criteria that a polyurethane foam should meet in pour in place foaming applications. One criteria is that an alternative blowing agent to ozone depleting CFCs and HCFCs is needed. A second requirement is that the polyurethane foam should flow well so that the entire cavity is filled with the foam. If the foam prematurely gels, voids will form behind the prematurely gelled foam where the foaming mass could not reach. A third requirement is to use the least amount of raw foaming material to fill a particular cavity to save on raw material costs. To adequately fill all portions of the cavity and prevent the presence of voids, it is often necessary to overpack the cavity. The less overpacking that is necessary to completely fill the mold, however, the greater the savings in raw material costs. Thus, it is desired to form a polyurethane-filled container having the lowest density possible.
Many polyurethane foam manufacturers are now turning to water as the sole source of blowing agent instead of CFCs or HCFCs. For example, in the field of cooling containers where the foam is poured in place, water-blown rigid polyurethane foams present a unique problem. Rigid polyurethane foams blown with water tend to be closed-celled foams which shrink and pucker over a period of time after foaming and during cure. This is partly due to the migration of carbon dioxide gas, produced by the water reaction with polyisocyanate, out of the closed cells and leaving behind a vacuum which then tightens and shrinks the foamed mass over time. Foam which shrinks in foamed-in-place applications will either pull away from a substrate, or continue to adhere to the inner surface of the substrates causing waviness and surface deformities on the substrate. The problem of foam shrinkage in CFC-blown and HCFC-blown foams has not been as acute since CFC gases tend to migrate out of the closed cells very slowly over a period of months or years, if at all, resulting in a minimized pressure gradient within the foam.
The problem of foam shrinkage or dimensional stability is more severe in applications such as picnic coolers where the coolers are often subject to wide temperature variations, from indoor 70.degree.-80.degree. F. temperatures to beach temperatures in direct sun which may climb to 110.degree.-120.degree. F., causing the gas in the cells to further expand and diffuse out.
It is also desired to produce a foam having a lower density yet which fully fills the cavity and is dimensionally stable to lower raw material costs. Lowering the density, however, especially in water-blown foam already having a tendency to shrink has the attendant disadvantage of further exacerbating the dimensional instability of the foam.
Traditional closed-cell water blown foam requires an in-place density of at least 2.4 pounds/ft.sup.3 (pcf) to possess enough polymer strength to withstand the tendency to shrink. As a result the use of all water blown foam has not been economically desirable since an HCFC blown foam can fill a cavity with dimensionally stable foam at about 2.0 pcf, resulting in a significant cost advantage for materials. Moreover, conventional closed cell water blown foam requires large amounts of more expensive high functional polyether polyol to provide the polymer cross linking necessary for dimensional stability.
Open celled foams have been described in U.S. Pat. Nos. 5,214,076; 5,219,893; 5,250,579; 5,262,447; 5,318,997; 5,346,928; and 5,350,777, each of which is incorporated herein in its entirety.